Cells infected with HSV-1 and HSV-2 (the facial and genital strains of HSV) are typically induced to express suicidal genes destined to destroy the cell prior significant viral replication. To overcome this effect and to secure the cell for viral replication, the virus undertakes an immediate counter attack by expressing the virion host shutoff (vhs) (UL41) gene—a 58 kDa structural component of the HSV-1 virion with a powerful mRNA destabilization/degradation activity. The vhs protein is shed into the cellular cytoplasm upon viral uncoating during viral entry into the cells (Read and Frenkel, 1983; Kwong et al. 1988; Kwong and Frenkel, 1987). Based on recent experiments, it seems that the vhs protein counteracts the cells' suicidal functions by immediate destabilization/degradation of the infected cell mRNAs, including house keeping genes and stress related suicidal genes induced post viral infection and may encode anti apoptotic genes. In consequence of the mRNA degradative activity host cell protein synthesis is shutoff, the suicidal proteins are not produced and the cells survive for a certain period of time, allowing viral replication before death of the target cell.
HSV-1 mutants carrying a mutation in the vhs gene have been developed. Whereas wild type HSV-1 infection is accompanied by host mRNA degradation HSV mutants which are deficient in the virion host shut-off (vhs) function (“vhs1 mutants”) allow continued cell protein synthesis. One such mutant termed UL41NAB was developed which infection into cells was shown to be attenuated in its ability to replicate and reactivate from latency (Leib, D., 1997).
HSV derived amplicons comprising at least one inserted gene under control of a promoter in association with helper HSV have been disclosed (Spaete and Frenkel, 1982, Frenkel et al., 1994, Vlanzy, D. A. et al., 1981). In one example of such systems, the associated helper virus is of a restricted replication competence in a normal host cell (Efstathiou S. et al., 1999). In another example, the recombinant HSV vectors are modified to target and infect a selected cell type (Spear, M. A. 2000).
The incidence of brain tumors is estimated to be 5-14.1 per 100,000. Gliomas account for 40-60% of the primary tumors, 75% of which are malignant. Gliomas are the most common primary tumor arising in the human brain. Malignant gliomas account for 30% of primary brain tumors in adults, and are divided by grade into two categories, anaplastic astrocytoma and glioblastoma. The estimated incidence of malignant glioma in the United States is 14.7 per 100,000, representing approximately 10,000-15,000 new cases annually. Despite improved aggressive surgical therapy, radiotherrapy and chemotherapy, malignant gliomas are almost always fatal; the overall 5 year survival rate for glioblastoma, the most malignant glioma, is less than 5.5% and the median survival is approximately 52 weeks. These figures have remained virtually unchanged over the past three decades. Treatment of systemic tumors often fails because of development of central nervous system metastases. The advanced stage indicates no curability. Most gliomas have poor prognosis without any completely effective treatment
Recently, HSV viral vectors were evaluated for their efficacy and safety in clinical use in humans. In one study, HSV derived vectors comprising HSV mutant viruses deficient in the gene encoding the 34.5 protein (a major determinant of neuropathology) were tested in patients with relapsed glioma. This mutant is a mutli-mutated conditionally replicating HSV vector termed “G207” which has deletions of both 34.5 loci and in the ICP6 (ribonucleotide reductase (RR)) which is required for replication in non-dividing cells (Rabkin, S. et al. 2000). The G207 mutant is now being tested in a Phase I Clinical Trial for recurrent glioblastoma in which it has been shown to be non toxic and without serious adverse events, but its efficacy has not yet been demonstrated. In addition, insertion of antineoplastic genes (specifically cytokine genes) into the mutated vector has been proposed (markert, J, 2001).
In another study an HSV type 1 thymidine kinase and gancyclovir gene therapy was evaluated as an adjuvant to surgical resection and radiation in adults with previously untreated glioblastoma multiform (Ranov, N. G., 2000). In a phase III trial patients with untreated glioblastoma multiform received either standard surgical and radiotherapy or standard therapy plus adjuvant gene therapy during surgery. Clinical safety of the treatment was determined and was comparable in both groups but there were no significant clinical differences between gene therapy and control patients.
HSV derived viral oncolytic vectors having high efficacy for treatment of human tumors yet maintaining their safety are desired.